Chu Fangfang, Heptner Nils, Lu Yan, Siebenbürger Miriam, Lindner Peter, Dzubiella Joachim, Ballauff Matthias
†Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany.
‡Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner Platz 1, D-14109 Berlin, Germany.
Langmuir. 2015 Jun 9;31(22):5992-6000. doi: 10.1021/la504932p. Epub 2015 Feb 9.
We study the structure and viscoelastic behavior of 3D plastic crystals of colloidal dumbbells in an oscillatory shear field based on a combination of small-angle neutron scattering experiments under shear (rheo-SANS) and Brownian dynamics computer simulations. Sterically stabilized dumbbell-shaped microgels are used as hard dumbbell model systems which consist of dumbbell-shaped polystyrene (PS) cores and thermosensitive poly(N-isopropylacrylamide) (PNIPAM) shells. Under increasing shear strain, a discontinuous transition is found from a twinned-fcc-like crystal to a partially oriented sliding-layer phase with a shear-molten state in between. In the novel partially oriented sliding-layer phase, the hard dumbbells exhibit a small but finite orientational order in the shear direction. We find that this weak correlation is sufficient to perturb the nature of the nonequilibrium phase transition as known for hard sphere systems. The discontinuous transition for hard dumbbells is observed to be accompanied by a novel yielding process with two yielding events in its viscoelastic shear response, while only a single yielding event is observed for sheared hard spheres. Our findings will be useful in interpreting the shear response of anisotropic colloidal systems and in generating novel colloidal crystals from anisotropic systems with applications in colloidal photonics.
基于剪切条件下的小角中子散射实验（流变小角中子散射，rheo-SANS）与布朗动力学计算机模拟相结合的方法，我们研究了胶体哑铃状粒子在振荡剪切场中的三维塑性晶体的结构和粘弹性行为。空间稳定的哑铃状微凝胶被用作硬哑铃模型体系，其由哑铃状聚苯乙烯（PS）核和热敏性聚（N-异丙基丙烯酰胺）（PNIPAM）壳组成。在剪切应变增加的情况下，发现了从不连续的类似孪晶面心立方晶体到部分取向的滑动层相的转变，中间存在剪切熔融状态。在这种新型的部分取向滑动层相中，硬哑铃在剪切方向上表现出小但有限的取向有序性。我们发现，这种弱相关性足以扰乱硬球体系中已知的非平衡相变的性质。观察到硬哑铃的不连续转变伴随着一种新型的屈服过程，其粘弹性剪切响应中有两个屈服事件，而对于剪切的硬球只观察到一个屈服事件。我们的研究结果将有助于解释各向异性胶体体系的剪切响应，并有助于从各向异性体系中生成新型胶体晶体，应用于胶体光子学领域。
